After thinking about it for years, I finally built a
(mostly) wooden “Steam” Engine. It actually runs off air (either blown as from
a hair drier, or sucked as from a shop vac), so it should more correctly be
called a pneumatic engine, but it illustrates the same principles as a steam
engine. Steam would be a bad idea here, as the moisture would make the wood
swell, and bad things would happen.

It is a modified version of the one done by Matthias Wandel
(http://woodgears.ca/air_engine/).
Or perhaps one might say it was inspired by his engine. I kept the same general
layout and scale as his, but I redesigned the valve assembly, and I used a
rather different
axle arrangement.

The main “non-woodenness” of it is that I used a bolt for
the axle rather than a dowel, and a pair of bearings to allow the bolt to
rotate. It was not feasible to offset the axle as in Matthias’s design, so I
used a more normal eccentric.

As illustrated below, the “cylinder” is actually
rectangular, and the valve body is formed by several layers of plywood screwed
on, each with one or more openings cut into it. It would have been neater to
use a router to make the channels, but I just used a drill press with a
Forstner bit and a lot of chisel work.

The two pieces of plywood above the valve slider (one of
which forms the side of the “cylinder”) contains a pair of air channels from
the ends of the “cylinder” to the middles of the slider, and a central channel
that forms the “outlet”. (Note that “outlet” and “inlet” are interchangeable.)

The two pieces below the slider contain a single channel
that directs air from the inlet to the ends of the slider.

The slider consists of two openings.
The operation is illustrated below:

When the valve slider moves to the left, air is directed
from the inlet to the left side of the piston, pushing the piston to the right,
and the air to the right of the piston is directed out the outlet. When the
slider moves to the right, things are reversed:

The opening for the slider is just a hair bigger than the
slider—only as much as will allow it to move easily. I trimmed the fixed side
pieces to be just thicker than the slider (maybe 0.005 inches?). Then I glued
and screwed one in place. I placed the slider and a piece of paper in place,
and then glued and screwed the other side piece in. After removing the paper,
and cleaning up any squeeze-out, I found that I had succeeded, and the slider
was tight but would slide easily.

For the piston, I cut three squares at the same time, and
drilled a hole through all three. The two end pieces in the stack became the
ends of the cylinder, and the center one became the piston. I lightly sanded
each of the pistons four edges, so that it could slide.

For the axle mechanism, I used a carriage bolt for the axle.
The head engaged the crank. Then after the first bearing I put the eccentric.
This is followed by the flywheel and the final bearing. I left the bolt
extra-long so that if I ever wanted to put something on to be driven by the
engine, I would have some axle to attach it to.

For the eccentric I wanted a bearing surface that was smooth
and round, and which preferably would have reasonably low friction sliding
against wood. Also, it had to be easily workable. I originally thought of using
a short length of PVC pipe, but then I ended up using an end cut off of some
sort of PVC coupling. It was cheaper and was a better diameter.

To make the eccentric, I laid out a circle on a wooden blank
the same size as the inside of the PVC. I located the center of this circle and
the offset circle for the axle. I drilled a small hole at the center and the
appropriately-sized hole (3/8”) for the axle. I then drilled a quarter-inch
hole radially from the outside into the axle hole (see the drawing below). I’ll
say what this hole was for shortly.

Then I roughed out the outer circle with a jig saw. I put a
small bolt through the center hole, chucked it in my drill press, and then
sanded it. I first got it round, and then I gradually reduced the size until it
just fit inside the PVC.

I needed a way to key it to the axle. Just snugging down a
nut on either side could still allow it to shift.

What I ended up doing was to use a 4-penny brad. I had a drill
bit just a tad larger than the brad. I figured out where on the bolt the
eccentric would sit, and then I drilled a small hole through the bolt, being
careful that the hole would be 90-degrees off of the crank angle. I also made a
similar hole where the flywheel would go.

Then I assembled onto the axle the crank, bearing, and
eccentric. I carefully oriented the eccentric to be 90-degrees from the crank,
and then I tightened the nut down on it. Once it was held in the proper
orientation, the hole in the axle was visible in the radial hole I had drilled
into the wooden blank. I used that hole as a guide to drill a hole through the
bolt and into the center of the blank. I could then slide a brad into that
hole, which would keep the eccentric from twisting on the axle. The PVC
cylinder would keep the nail from falling out.

I did something similar for the flywheel, except that I
didn’t have access to the side, like for the eccentric. Here I just put a nail
halfway through the hole in the axle, so that it stuck out both sides. I put
the flywheel against it and traced out the nail. Then I used a jig saw
to
cut a slot
on both sides of the central hole of the flywheel, to line up with the
nail. This allowed me to slide the flywheel over the nail, which then prevented
the flywheel from slipping with respect to the axle.

The follower for the eccentric wasn’t as easy. The size of
the PVC was between two of my Forstner bits, so I couldn’t just drill it
directly. I had to drill it one size smaller and then sand it to make it
larger. I couldn’t spin it to make sure that it was staying round, so I just
had to be careful and keep trying the fit.

Most of the junctions (e.g. at the end of the crank) are
screws, where I used a long enough screw that the rotating part did not have
any threads on them. After screwing the screw to the proper depth, I then
removed the screw, cut off the excess length, and screwed it back (now that the
threads had already been cut).

The exception was the piston rod to crank shaft. That was
just a cut down 4-p nail. As I mentioned previously, one of my drill bits was
just a tad larger than the nail, and the next smaller size was a tad small. If
I had drilled all of the holes with the larger bit, then the nail would be
loose, and I would afraid that it would fall out. So I made the outer holes (on
the fork) with the larger bit, and then drilled the inner hole with the smaller
bit. That hole was a bit too small to get the nail through, so I cut off the
head of a nail, chucked it into my drill, and “drilled” the nail through the
smaller hole. This enlarged it enough that I could push a nail through it, but with
enough resistance that it wouldn’t fall out.

The rest of the construction was fairly straight-forward. I
first attached the axle to the base, and then carefully positioned the
piston/valve assembly so that the valve slider was centered, i.e. so that as
the eccentric turned, the slider moved the same distance away from the center.
Then I measured and drilled the piston’s connecting rod so that the piston
would be roughly centered in the cylinder. It wasn’t as critical that the
piston be centered as that the valve slider be centered.

I shouldn’t have been, for some reason I was surprised when
I first put the shop vac nozzle in place, and the engine actually ran.

I found that it actually ran at a good clip. I don’t have an
accurate way to measure its speed, but looking at video footage seems to show
that it runs between 900 and 1000 RPM.

One issue is that the engine is totally unbalanced. So with
the piston going back and forth 15 times a second, it vibrates rather a lot. I
thought about adding some counterweights to the flywheel, but particularly as
the flywheel is on the other end of the axle, I’m not sure how well that would
work.

Given that this project was an engine, and the purpose of an
engine is to move, I figured that still pictures would not do it justice. So
here is a short video of it in operation.